Systems for protecting guarded areas and detecting intruders are known in the art. One example of a intruder detection system is an electrical or electronic fence which provides an alarm whenever there is an intrusion, since such a change indicates that the fence has been touched. Such systems only detect intrusion when the intruder enters or is about to cross a boundary or a guarded perimeter.
Various existing security systems are discussed in pages 33-41 of an article entitled "Was kann man erwarten--was ist zu beacten?" by Bernd Bull , published in vol 19, no 4 of the "Protector", September 1991 in Zurich.
Other detection systems use cameras which continually view the secured area. Any change in the view is visible to an operator providing he is continuously watching the screen. Such systems allow the operator to distinguish between the actual view and a pre-recorded view. Furthermore, the cameras need to be switched on all the time and during the hours of darkness require relatively expensive lighting or special night cameras.
Other detection systems actively search for intruders by continually scanning the area to be guarded. One example of a scanning system seeks to detect objects which are not among the known objects of the background or movement at a significant rate. If an intruder has been detected, the scanning systems typically track the location of the intruder.
Japanese Patent No. A 8055286 to Kawamoto Yuichi describes an active infra-red sensor device using a plurality of sensor poles so that the infrared rays emitted by the infrared light source of one sensor pole are detected by the next sensor pole. A disadvantage of active infra-red sensor devices is that they can only detect objects which break the beam emitted by the infrared light source between the transmitting and receiving sensor poles. Thus, to detect objects outside a perimeter boundary, the sensor poles also need to located outside.